CityGML is a data model used to represent the geometric and semantic information of urban 3D city objects. There are several ways to generate 3D models for applications such as gaming, media content for movies and TV, and 3D printing among others. Since visualization is the primary purpose of these methods of 3D model generation they lack the semantic information required for applications such as spatial data-mining, thematic queries, and geospatial simulation and analysis. Therefore, there is a need to develop methods for mapping sematic information of new and existing 3D geometric models and their conversation to CityGML formats so it can be stored for future use. The conversion of must allows mapping of classes of objects with a higher Level-Of-Detail (LOD) such as LOD3 and LOD4 which represent an urban city model in greater detail. A methodology of the conversion is developed and a prototype is implemented to validate the algorithms and the methods developed for this project. The methods can be broadly categorized into three parts a) Extraction of geometric information; b) Segmentation of the model into various groups for semantic mapping; c) Mapping of semantic information on the model. Topological aspects are considered for the mesh segmentation to allow users to easily select and map semantic information to the geometric model. The generated models can be used in a wide range of applications ranging from disaster management to urban simulations such as rooftop solar panel installations.